Method for gauging surfaces with classification of measurements as valid or non-valid

ABSTRACT

A method for measurement of a surface, in particular according to the principle of Optical Coherence Tomography, whereby distances to points of the surface are measured based on interferograms and classifying of measurements as non-valid or valid based on evaluation of phase change or amplitude change of a respective interferogram.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority to European Patent Application No.1915606.4, filed on Feb. 7, 2019. The foregoing patent application isherein incorporated by reference.

FIELD OF THE INVENTION

The invention relates to a method for gauging surfaces and a measuringdevice for this purpose.

BACKGROUND

In many fields of use, there is a need to gauge surfaces of objects andhence also the objects themselves with high accuracy. This applies inparticular to the manufacturing industry, for which the gauging andchecking of surfaces of workpieces is very important.

For these applications, there is a number of existing measuring deviceswhich are designed for specific tasks and are also designated ascoordinate measuring devices or machines. These measuring devices gaugethe surface by producing mechanical contact and probing the surface.Examples of this are gantry measuring machines, as described, forexample, in DE 43 25 337 or DE 43 25 347. Another system is based on theuse of an articulated arm whose measuring sensor arranged at the end ofthe multipart arm can be moved along the surface. Articulated arms ofthe generic type are described, for example, in U.S. Pat. No. 5,402,582or EP 1 474 650.

Approaches for non-contact gauging have already been pursued in theprior art. One approach utilizes white light interferometry forhigh-precision gauging. Here, the application either employs scanning,i.e. by displacement of the interferometer, and therefore takes placeslowly or, in the case of spectrally resolved detection, as a rule withlimitation to a measuring range of a few mm.

EP 1 744 119 discloses a system for gauging surfaces using opticalcoherence tomography and a frequency-modulated source. Here, a fibrering laser is made tuneable by an acoustically tuneable filter element.The laser radiation is then used for interferometric gauging of surfacesin a common path interferometer, i.e. an interferometer which uses atleast partly the same components or beam paths for measuring radiationand reference radiation. The reference distance here is provided by areflection in the measuring arm of the interferometer. A calibrationinterferometer is used for calibrating the wavelength.

There are different problems associated with such interferometricmeasurement methods and devices. An important source of error is theoccurrence of so called speckles. The speckle effect originates from thecoherent superposition of light with different relative phases reflectedfrom the surface within the resolution cell of the scanning device. Suchphase differences occur for example from rough surfaces where the heightvariations are on the scale of the used wavelength and the lateral sizeis smaller than the resolution cell. Due to the disturbing speckleeffect, the measured profile shows outliers which are not present in thereal surface profile. Known methods and devices for interferometricmeasurement of surfaces are not able to adequately deal with theseerrors.

BRIEF DESCRIPTION

An object of some embodiments is to provide an improved measuring methodor measuring device for gauging surfaces or for determining surfacetopographies.

A further object is to provide a measuring method or measuring devicewhich adequately deals with, in particular speckle induced, measurementdisturbances.

These objects are achieved by the subjects of the independent claims orof the dependent claims, or the solutions are further developed.

Some embodiments of the invention pertain to a method for, in particularindustrial, measurement of a surface, preferably according to theprinciple of Optical Coherence Tomography, whereby based oninterferograms, e.g. by analyzing the respective modulation frequency,distances to points of the surface are measured. The method comprisesgenerating a laser beam, emitting the laser beam onto the surface,whereby the laser beam is focused on a point of the surface, receivingat least a part of the laser beam, reflected by a respective point ofthe surface, and generating an interferogram by superposition of thereceived laser beam with a reference laser beam.

The method further comprises the step of classifying of measurements asvalid or non-valid based on evaluation of phase change and/or amplitudechange of a respective interferogram. Said otherwise, a respectiveinterferogram or the distance derived therefrom are classified as validor non-valid based on a test of phase and/or amplitude change of therespective interferogram. Preferably, the classification serves forsorting out or tagging/marking of measurements disturbed by occurrenceof laser light speckles.

Preferably, measurements classified as non-valid are tagged and storedas non-valid or deleted in real-time during measurement. Optionally, arespective interferogram is tagged or deleted before any processing fordistance calculation, i.e. non-valid measurements are recognized in duetime before any further processing is effected which would be “waste” ofprocessing power as the result is too flawy anyway. As another option,the method comprises generating a profile of the measured surfacewherein non-valid measurements/distances are graphically marked.Alternatively or additionally, non-valid distances are excluded from theprofile, wherein preferably continuity of the profile is maintained byinterpolating between non-excluded measurements.

Optionally, a respective measurement is classified as non-valid if theresult of the evaluation is above one or more defined thresholds,wherein preferably the threshold is defined in a calibration procedurewith measuring of one or more standard surfaces. For example, if thephase change of a respective interferogram exceeds a defined threshold,the respective interferogram is categorized as non-valid.

As another option, the evaluation comprises searching for a disturbanceof phase and/or amplitude of a respective interferogram. Alternativelyor additionally, the evaluation comprises determining a degree offluctuation of the phase and/or amplitude of a respective interferogram.Said otherwise, it is evaluated how much phase and/or amplitudevariation or drift is present in a respective interferogram.

As another option, the evaluation comprises comparing a phase and/oramplitude chart of a respective interferogram with an ideal phase and/oramplitude chart. The charts can be embodied as graphs, functions, tablesetc. as form of data embodiment for derivation of a deviation of themeasured phase and/or amplitude from ideal values.

In a preferred embodiment, the evaluation comprises calculating theunwrapped phase of a respective interferogram, fitting a linear functionthrough the interferogram phase (chart), subtracting the linear functionfrom the phase, calculating the Standard Deviation and classifying theinterferogram based on the Standard Deviation. If for example theStandard Deviation exceeds a predefined threshold, the respectiveinterferogram or measured distance is classified as non-valid.

In another preferred embodiment, the evaluation comprises detecting ifthe amplitude of a respective interferogram is temporarily below anamplitude threshold. The amplitude threshold can be an absolutethreshold. Alternatively, the threshold is a relative one, depending ona maximum amplitude of a respective interferogram. As a further option,a respective measurement is classified as non-valid if an interferogramfraction with amplitude below the amplitude threshold is above afraction threshold. Said otherwise, according to this further option, itis not only evaluated if there is amplitude below the amplitudethreshold is present in the interferogram, but it is also evaluated towhat extent such low amplitude is present. If for example, the lowamplitude is detected but it lasts not longer than a predefined period,the measurement is classified “valid”.

Optionally, an amplitude based weighting factor for phase information ofa respective interferogram classified as valid is applied forcalculating the distance to the point. As a further option, theweighting factor is directly dependent on the amplitude of a respectiveinterferogram and/or is set as zero if the amplitude is below anamplitude threshold. The amplitude threshold for phase weighting may be(but has not to be) identical to the above mentioned amplitude thresholdfor classification.

Some aspects of the invention also pertain to an interferometricmeasuring device designed for measuring a surface, in particularaccording to the principle of Optical Coherence Tomography, the devicecomprising a laser for generation of a laser beam, a drive for guiding alaser beam emitting measurement head above the surface such that thelaser beam is scanning the surface point-by-point, a receiver forreceiving at least part of the laser beam reflected by a respectivepoint of the surface and an interferometer for generating aninterferogram by superposition of the received laser beam with areference laser beam as well as a signal processor for measuring adistance to a respective point based on a respective interferogram.

According to some embodiments of the invention, the signal processor,e.g. a Field Programmable Gate Array (FPGA), is configured to classifymeasurements as valid or non-valid based on an evaluation of phasechange and/or amplitude change of a respective interferogram.

Additionally, some embodiments of the invention also pertain to anon-transitory computer program product, comprising program code whichis stored on a machine-readable medium, in particular of aninterferometric measuring device according to the invention, and havingcomputer-executable instructions which when executed cause a computer toperform the measurement method according to the invention.

Some aspects of the present invention allows advantageously to identifyand sort out interferograms resp. distances derived therefrom which have(too much) errors, in particular errors resulting from speckles. Thus,invalid measurements are dismissed from the beginning and do not have tobe erased afterwards. A resulting measured surface profile does not showany (speckle induced) outliers like resulting from methods/devices ofthe prior art resp. such outliers can already be marked as flawymeasurements during creation of the profile.

BRIEF DESCRIPTION OF THE DRAWINGS

A method according to some aspects of the invention and a measuringdevice according to some aspects of the invention for gauging surfacesare described or illustrated in more detail below, purely by way ofexample, with reference to working examples shown schematically in thedrawing. Specifically,

FIG. 1a-f show a first example of an interferometric measuring deviceand an according method for measuring of a surface with classificationof measurements as valid or non-valid;

FIG. 2a,b show another example of a method for classification ofinterferograms;

FIG. 3 shows a further development of a method for classification ofinterferograms; and

FIG. 4 shows another further development of a method for classificationof interferograms.

DETAILED DESCRIPTION

FIGS. 1a-f illustrate a first example of an interferometric measuringdevice 1 and an according method for measuring of a surface 24, wherebyin the example, the measurement is based on the principle of SweptSource Optical Coherence Tomography (SS-OCT). Although the herewith inmore detail described device and method are based on swept-source OCTthe following aspects are applicable also for Fourier-domain OCTconsisting of a white light source in combination with a spectrometerand line sensor on the detection side. The device 1 is designed forusage in the intended field of industrial coordinate measuring deviceswith free-beam measurements of a few cm using compact probe heads havingdiameters in the region of ruby spheres. In this frequency-modulatedinterferometry, a source which should as far as possible permitbroadband tuning in a short time is used. Moreover, narrow-bandcharacteristics with a coherence length of a few cm are required. Thetuning of the source is e.g. referenced via a calibration interferometerwhose length is known very precisely.

In FIG. 1a , a frequency modulated laser (swept source) 20 generates alaser beam 23 which is guided by optical fibers 26 to a measurement head25 and emitted therefrom at the surface 24 to be measured. The lasersource 20 is for example in the form of a fibre ring laser having anoptical semiconductor amplifier as an amplifying medium and a tuneablefilter element. If higher repetition rates are desired in themeasurement, the fibre ring laser can be extended by a fibre length ofseveral kilometres, the repetition rate corresponding to the inverse ofthe transit time of the light in the fibre ring. As a furtherpossibility for setting up the laser source 20, it is also possible touse an external cavity having a dispersive element, for example agrating or prism in combination with a moveable optical surface, e.g. apolygonal mirror, for fast tuning of the laser wavelength for the laserresonator. The tuneable element may be formed, for example, as aFabry-Perot filter or as an acoustically tuneable fibre Bragg grating.Further elements are Semiconductor Lasers, tunable VCSELs (Verticalcavity surface emitting laser), distributed feedback lasers (DFBs),optical couplers or insulators, the use and integration of which in sucha system are known to the person skilled in the art.

The laser beam 23 is focused at a spot or point on the surface 24. Theback reflected light is superimposed with light from a fixed reflectorwhich serves as a local oscillator (e.g. the last optical surface of themeasurement head 25). Due to the modulation, light with differentfrequencies interferes depending on the distance D to the object 24 andthe associated time delay. This results in a temporal amplitudemodulation or interferogram whose frequency (beat frequency) is directlyrelated to the distance D to a respective point of the surface 24.

The resulting temporal amplitude modulation or interferogram is detectedby a photo detector 21 and converted to a digital signal resp.digitalized interferogram 2 (see also FIG. 1b ). Such an interferogramcan be (completely) continuous. Alternatively, an interferogramcomprises a couple of discrete segments, for example in devices with anDFB-array as swept light source. Each interferogram segment or“sub”-interferogram is assigned to the tuning-region of a particularDFB. In any case, for subsequent evaluation (classification ofinterferograms/measurements) as described below, a respectiveinterferogram is considered as a whole, wherefore for example in case ofdiscrete segments, the segments are stitched together before evaluation.Said otherwise, the signals e.g. from different tuning-regions are firstcombined.

A signal processing unit 22, e.g. a FPGA or some other form of computerprocessor, provides the calculation of the distance D to the targetsurface 24 (more precisely: the targeted surface point) by analyzing themodulation frequency of the respective digitalized interferogram. Inother words, the calculation of the distance D is primarily based on thephase information of a respective interferogram 2.

By moving the laser spot over the surface 24 i.e. scanning (arrow 28), aplurality of surface points and thus the height variation or profile ofthe surface 24 is measured. However, the measurement can be disturbed,i.e. the measured distance deviates from the real distance D, which willbe explained in more detail with respect to FIG. 1 b.

Referring to FIG. 1b , a measurement disturbance caused by speckles,i.e. position-dependent intensity variations, in coherent observation,is described. In the upper part of FIG. 1b , it is shown that the spotof the laser beam 23 on the surface 24 has always a certain size whichcorresponds to the resolution of the sensor defined by the PSF(point-spread-function) of the lens. In case of a rough surface 24,height variations can occur within the resolution cell. The light 23 rreflected from different parts of the illuminated area is sent to thedetector 21 with different phase (in the example two phases P1 and P2are illustrated). Depending on their relative phase the light caninterfere constructively or destructively. Furthermore, the phase of thesuperimposed light 23 r can differ significantly from the phase of thesingle parts. Said otherwise, this so called speckles originate from thecoherent superposition of light 23 r with different relative phase P1,P2 reflected from the surface 24 within the resolution cell of thescanning device. Such phase differences occur for example from roughsurfaces 24 where the height variations are on the scale of the usedwavelength and the lateral size is smaller than the resolution cell.

The resulting interferogram 2 (lower part of FIG. 1b ), outputted by thedetector 21 is disturbed (area graphically marked by circle 29) andaccordingly, the signal processor 22 calculates (or would calculate) awrong distance. With the presented invention, such disturbedmeasurements are recognized which is exemplified with respect to FIG. 1c.

FIG. 1c shows an exemplary procedure for classification 7 ofmeasurements as valid or non-valid. First, after generation of theinterferogram 2, the phase of the interferogram is calculated (step 3).Thereafter, phase change is determined (step 4). The phase change 4serves as indicator of disturbance. If the phase change 4 is above acertain threshold (tested in evaluation step 5), the interferogram resp.the measurement (signal) is classified as non-valid (6 a), i.e. (toomuch) disturbed. Otherwise, the measurement is classified as valid (6b). Such a procedure is further exemplified with respect to FIG. 1 d.

FIG. 1d shows in the upper part a chart of calculated (unwrapped) phaseD (displayed over time) of a measured interferogram 2. Ideally, withoutany disturbance, the phase line 13 would be a straight line. In case ofa speckle disturbance the phase 13 deviates from the ideally linearfunction (marked by circle 29). The phase change 4 is determined byfitting a linear function 12 through the interferogram phase 3 resp.phase line 13, subtract the linear function 12 from the phase 13 andcalculate the Standard Deviation of the phase D, this is a measure forthe deviation (indicated by arrow 4) of the phase 3 from a linearfunction 12. By calculation of the Standard Deviation, in this examplethe phase change is determined, i.e. the Standard Deviation serves as ameasure for phase change. Instead of using functional representations ofthe phase as shown, the comparison of the measured chart with an idealchart is based on tables.

If the Standard Deviation 4 is larger than an adjustable threshold, thenthe interferogram 2, and also the resulting distance value, areclassified or recognized as “invalid”. The value of the threshold cane.g. be determined by scanning tests on a typical rough surface 24.Measurements classified as non-valid are tagged as non-valid ordismissed (deleted), preferably in real-time during measurement, i.e.either stored with a marker or completely removed, which is exemplifiedin more detail with respect to FIG. 1 e.

FIG. 1e shows an exemplary final result of the measurementclassification. The measured distances D or the measured profile 10 areshown, (grey or bright line). Due to the disturbing speckle effect, themeasured profile 10 shows outliers 9 which are not present in the realdistances resp. the real surface profile 10′ (black or dark line). Suchoutliers 9 have a typical shape similar to a pole in mathematicalfunctions.

As these disturbances have been recognized by the previous evaluation 5resp. the according measurements have been classified as non-valid, thedistance values which are declared as “invalid” can be marked in theprofile plot, indicated in the figure by dots 11. If for exampleroughness parameters like Ra or Rz shall be determined from the profile10, those invalid points can be excluded from the calculation. Anotherpossibility is to interpolate the profile 10 between adjacent “valid”distance points in order to obtain (or maintain) a continuous profile 10without speckle disturbances, which is in FIG. 1e indicated by the thickstraight line 8. In embodiments of the method wherein non-validmeasurements or distances are not marked in the surface plot, it isoptionally waived to process the underlying interferograms 2 at allafter they are classified as non-valid. Hence, they are not subject todistance calculation at all and optionally even completely removed bythe processing unit, which can spare processing time and power and incase of complete and quick deletion save storage.

In either way, advantageously, the disturbances, primarily those byspeckles, do not effect the final measurement result 10. Hence, thepresented method provides analyses of phase change as a quality markerto find and tag distance values of disturbed interferograms.

FIG. 1f shows an additional optional step. The figure is based on FIG.1e and shows additional measurement points 11 a “before” and “after” theprevious invalid measurement points 11 are considered “invalid” thoughthey have not been classified as “invalid” in the previous steps. Indifference to the previous invalid points 11 a, these additional invalidpoints 11 a are considered as such because of their neighborhood to theinitial invalid points 11. Said otherwise, the invalid region of theoriginal points 11 is “artificially” broadened with the additionalpoints 11 a. This optional step serves for example to guarantee that aspeckle underlying the disturbance is completely considered, without anyboundary speckle zone omitted.

As criterion, for example each 10, 100 or so measurements “before” and“after” the original “invalid” measurements is declared “invalid”, too,or a margin of the original invalid region is declared “invalid, e.g. 1%or 2.5% of the invalid profile at one or each end.

In accordance, a larger interpolation zone 8 a than in the previous FIG.1e is optionally established, comprising both the zone of values 11initially classified as “invalid” as well as the zones of values 11 aconsidered “invalid” due to their proximity to the initial values 11.

FIG. 2a shows another example of a method for classification 7 ofinterferograms 2. In a step 3′, the amplitude of the interferogram 2 iscalculated. Next, in step 4′, any change of amplitude is determined.Amplitude fluctuation serves as indicator for validness. If the changeor variation is above a certain threshold, tested in step 5, then theinterferogram 2 is classified as non-valid (step 6 a). Said otherwise,if a too high fluctuation of amplitude is detected, e.g. a too strongdrop of amplitude, then the respective measurement is declared“invalid”. If on the other hand there is no drift above the threshold,then the interferogram 2 is regarded as valid (step 6 b).

A test-threshold can be completely predefined, i.e. a value ispredetermined as such. Alternatively, the test-threshold issemi-predefined in depending on a measured value of the interferogram,e.g. the maximum amplitude present in the interferogram or the generaldegree of phase change (gradient of line 12 in FIG. 1d ). For example,the threshold is semi-predefined as a change which amounts to 25% or 50%of the maximum amplitude.

FIG. 2b shows a further exemplified illustration of an amplitude basedclassification. Shown is the amplitude A of an interferogram to beclassified. The amplitude A is not unvarying but there is significantchange (here in the middle, indicated by region B). Such a temporallyamplitude drop down is e.g. caused by speckles. The interferogram showsthat there is amplitude as low that it falls below a defined amplitudethreshold 14 which serves as an “inverse” amplitude change threshold(i.e. if there is amplitude below the amplitude threshold 14, this isregarded as exceeding an according amplitude change threshold). Thus,the interferogram resp. the measurement is classified as non-valid.

Instead of an amplitude value as a threshold 14 as shown, e.g. athreshold test based on a standard deviation of the measured amplitudechart 19 to an ideal amplitude chart is effected, comparable to themethod as shown in FIG. 1d . As another option, not the curve 19 but itsderivative is compared to a threshold, e.g. the gradient must not exceeda certain value.

In another procedure, illustrated by FIG. 3, the classification 7comprises not only a test if there is amplitude below an amplitudethreshold (14 in FIG. 2b ) but additionally it is tested if such lowamplitude section (B in FIG. 2b ) accounts for a too big part of thewhole interferogram. FIG. 3 is based on FIG. 2a whereby steps 2 and 3′are not shown for reasons of more compact illustration.

As in FIG. 2a , in step 4′ any amplitude change is determined and instep 5, it is verified if there is amplitude change above an amplitudethreshold. If “no”, the measurement is classified as a valid measurement(6 b).

If there is change above the first threshold, then it is furtherevaluated if the fraction or portion of amplitude change above the firstthreshold is above a second threshold/fraction threshold (step 15). Onlyif the second threshold is exceeded, the measurement is classified asnon-valid (6 a). Otherwise, the amplitude change, although above theamplitude threshold, is regarded as not rendering the measurementinvalid (step 6 b′).

Said otherwise, it is not only looked if there is significant amplitudechange but also if in the case of such major amplitude variation thischange concerns at least a predefined portion of the interferogram. Onlyif high amplitude change is detected that prevails a defined period,then the interferogram is classified as non-valid (6 a). Said the otherway round, if amplitude change above the first threshold is present butthe change lasts only a period shorter than a predefined period, themeasurement is still regarded as valid as the amplitude change issignificant but concerns only a portion of the interferogram which isregarded as a negligible portion.

With respect to FIG. 2b , the method according to FIG. 3 is furtherexemplified. In the example of FIG. 2b , region B shows amplitude belowthe amplitude or first threshold 14 as described above. Regions A and Cshow amplitude values above the first threshold 14. Then, the fraction Fof amplitude change above the first threshold 14 is calculated asF=size(B)/size(A+B+C).

If the fraction F is above a defined fraction threshold, then theinterferogram is classified as non-valid. In other words, if the size Bamounts to more than a defined portion of the size of the wholeinterferogram, the interferogram is tagged as invalid.

The values of the described thresholds are e.g. determined by scanningtests on a typical rough surface. The described procedures areoptionally combined to achieve higher robustness, e.g. there is testwith respect to phase change and additionally test with respect toamplitude change. Both evaluations can be performed independently and ifeither one of both results in “non-valid”, the measurement is classifiedas “non-valid”. Or, these test are performed in sequence, for examplefirst there is a test for amplitude change as described in FIG. 2b , andinterferograms 2 classified as “non-valid” because of amplitude changeare tested for phase change. If the phase change results in “non-valid”classification, too, the measurement is finally classified as“non-valid”.

Otherwise, the measurement is classified as “valid”. As an alternative,in case of divergent classification results with respect to amplitudeand phase, amplitude and/or phase classification is done a second time,this time e.g. with more refined thresholds.

FIG. 4 illustrates a further development of the method of classification7 of interferometric measurements. FIG. 4 is based on FIG. 1b or 2 awhereby steps 2, 3/3′ and 4/4′ are not shown for reasons of more compactillustration. In this further development, the detection of phase and/oramplitude disturbances in the interferograms is not only used to declarean interferogram, and also the resulting distance value, as invalid. Itis also used to improve the robustness of “valid” distances, e.g. incase of (weaker) speckles disturbances.

In this further development according to FIG. 4, after step 5 whereinthe phase and/or amplitude change is used for classification ofinterferograms, a respective interferogram classified as non-valid (step6 a) is dismissed or ignored (step 16). That means an interferogramrecognized as non-valid is not used for calculation of a distance D to apoint of the measured surface.

On the other hand, an interferogram classified “valid” is furtherprocessed in that the phase information is weighted, based on theamplitude of the respective interferogram (step 17). This isparticularly advantageous in classifications 7 that are based onamplitude change evaluation (e.g. as described with respect to FIG. 2a,b) as in these cases the amplitude is calculated/processed anyhow. Withthe time dependent weighting factor w(t) the phase information Φ_(proc)to be used for the distance processing is calculated from the raw phaseinformation Φ_(raw) according to:Φ_(proc)(t)=w(t)Φ_(raw)(t)

The weighted or processed phase information Φ_(proc) then is used forcalculation of the distance D to the underlying surface point (step 18).

Different weighting methods are applicable, whereby the weighting factoris preferably directly derived from the amplitude. As one option, theweighting factor is directly derived from the amplitude in that theamplitude itself is used as a weighting factor for the phase:w(t)=A(t)

Another option for directly deriving the weighting factor from theamplitude is to use the power k of the amplitude as a weighting factorfor the phase:w(t)=A(t)^(k),whereby k is e.g. a real number smaller or greater than 1.

As a further option, phase information is only taken into account if therespective amplitude is above an amplitude threshold (e.g. threshold 14in FIG. 2b ). Said otherwise, phase information is excluded fromdistance processing where the amplitude is below the amplitudethreshold.

For example, regions of “valid”-interferograms below the above describedamplitude threshold (e.g. region B in FIG. 2b ) are excluded fromcalculation of the distance (or only used with a lower power k than forthe other amplitude regions, e.g. regions A and C in FIG. 2b ). Saidotherwise, e.g. in a classification 7 as shown in FIG. 3, the gainedknowledge about the fraction F/region B is used to eliminate (or atleast diminish) the influence of the respective phase information oncalculation of the distance to the surface point for “valid”interferograms showing such (temporarily minor) amplitude fluctuation.

The combination of this phase weighting method with the speckledetection for a whole measurement sweep is particularly advantageous.For this, strongly disturbed interferograms are tagged as “invalid”distances. Those distances can be excluded or interpolated like forexample described with respect to FIG. 1e . The calculation of theremaining “valid” distances make use of one of the phase weightingmethods described above which reduces the disturbing effect of weakerspeckles. This approach drastically increases the overall robustness ofthe profile measurement.

A skilled person is aware of the fact that details, which are here shownand explained with respect to different embodiments, can also becombined in other permutations in the sense of the invention if notindicated otherwise.

What is claimed is:
 1. A method for measuring a surface, the methodcomprising: generating a laser beam; emitting the laser beam onto thesurface; receiving at least a part of the laser beam, reflected by arespective point of the surface; generating an interferogram bysuperposition of the received laser beam with a reference laser beam formeasuring a distance to a respective point based on a respectiveinterferogram; and classifying the respective interferogram or ameasured distance derived therefrom as non-valid or valid based on anevaluation of a phase change or an amplitude change of the respectiveinterferogram.
 2. The method according to claim 1, wherein the measureddistance is classified as non-valid if the result of the evaluation isabove one or more defined thresholds.
 3. The method according to claim1, wherein the evaluation comprises searching for a disturbance of thephase or the amplitude of the respective interferogram.
 4. The methodaccording to claim 1, wherein the evaluation comprises determining adegree of fluctuation of the phase or amplitude of the respectiveinterferogram.
 5. The method according to claim 1, wherein theevaluation comprises comparing a phase chart or amplitude chart of theinterferogram with an ideal phase chart or amplitude chart.
 6. Themethod according to claim 1, wherein the evaluation comprises:calculating the phase of the respective interferogram, fitting a linearfunction through the phase, subtracting the linear function from thephase, calculating a Standard Deviation of a result of the subtractingthe linear function from the phase, classifying the measured distancebased on the Standard Deviation.
 7. The method according to claim 1,wherein the evaluation comprises detecting if the amplitude of therespective interferogram is temporarily below an amplitude threshold. 8.The method according to claim 7, wherein the measured distance isclassified as non-valid if an interferogram fraction with an amplitudebelow the amplitude threshold is above a fraction threshold.
 9. Themethod according to claim 1, further comprising: calculating thedistance to the point by applying an amplitude based weighting factorfor phase information of the respective interferogram which has beenclassified as valid.
 10. The method according to claim 9, wherein theweighting factor: is directly dependent on the amplitude, or is set aszero if the amplitude is below an amplitude threshold.
 11. The methodaccording to claim 9, wherein the measured distance which has beenclassified as non-valid is tagged and stored as non-valid or deleted inreal-time during the classification.
 12. The method according to claim1, wherein the classification serves for sorting out or tagging ofmeasurements disturbed by occurrence of laser light speckles.
 13. Themethod according to claim 1, further comprising: generating a profile ofthe measured surface wherein a non-valid interferogram: is excluded fromthe profile, or are graphically marked.
 14. A non-transitory computerprogram product, comprising program code which is stored on amachine-readable medium of an interferometric measuring device andcarries out the method of claim
 1. 15. An interferometric measuringdevice designed for measuring a surface, the device comprising: a laserfor generating a laser beam; a drive for guiding a laser beam emittingmeasurement head above the surface such that the laser beam scans thesurface point-by-point; a receiver for receiving at least part of thelaser beam reflected by a respective point of the surface; aninterferometer for generating an interferogram by superposition of thereceived laser beam with a reference laser beam; a signal processor formeasuring a distance to a respective point based on a respectiveinterferogram and classifying the respective interferogram or themeasured distance derived therefrom as non-valid or valid based on anevaluation of a phase change or an amplitude change of the respectiveinterferogram.